CDL Air Brake Valves
Air brakes valves control, regulate and direct the flow of air through the system.
A water tap, for example is a valve that controls the flow of water into a sink.
The brake pedal, like the water tap, controls the flow of air to the service brake chambers, thus controling the brakes as you're going up and down the road.
See these articles for more information about the air brake valves so that you pass your CDL air brakes first time.